No you don't and no, it doesn't.
There is no reason to believe it should be any, other way.

Now, if you want to know what is going wrong:
-I assume you are talking about damage-tests in the garrison, correct me if wrong.
-Your loadouts are completely identical and you confirmed that with the other player, piece by piece
-You deal the same damage on tyranid-dummies, just to confirm your loadouts are completely identical
-You both fire full-auto/single-auto and you both aim for the lower torso to avoid headshots
-You both have a latency of 60 or less

Different rate of fire may simply be perception as the netcode of the game doesn't exactly do a stellar job of giving the correct audio-cues for hits taken etc. And it just gets worse with latency. A laggy player may fire off 20 shots before the game even tells the victim that player has started shooting, thus all the damage up to that point will get delivered in a "bundle" rather than as individual hits.

If you're familiar with Battlefield 4's netcode issues (and by proxy Hardline), you'll know it suffered from that, heavily, and players would fire off the amount of shots needed to kill someone and the player only received one hit worth of cues. But in some cases you could record the health-box on the HUD at really high FPS and see it ticking down in stages, but so fast that it was 100->75->50->25->0 in about 33 milliseconds, and other times it happened faster than that due to latency, making the game only display 100->0 (even though on the shooter's end, it took them 2-3++ seconds of continuous shooting before the victim died).

But I have notice the last few days, it sounding like bolters are shooting at different rates of fire at times, and I have no idea wtf is causing it. Someone was next to me with a normal bolter shooting, and it sounded like a stormbolter/combi-bolter

Thanks For response but i and my friend established that this is netcode.
so Results:
------------------------------------------------
eu server my ROF
i see: 333 friend see 333 (60 ping)
later
i see 333 friend see 250 (+\-) (my 60 his 76-9 ping)
American Server:
i see 333 friend see 666 (+\-) (my ping 80 his 100+)
later
i see 333 friend see 333 (my ping 87 his 160+)
------------------------------------------------
that's netcode problem sometimes happend that i should kill him her took cover and then died.
etc. so i say that is netcode problem,

I noticed that when I'm shooting someone I hear a whole lot more shots going out than when I'm being shot at. It's like say I shoot 10 rounds, to the enemy it would sound like 3 rounds but actually be 10.

No you don't and no, it doesn't.
There is no reason to believe it should be any, other way.

Now, if you want to know what is going wrong:
-I assume you are talking about damage-tests in the garrison, correct me if wrong.
-Your loadouts are completely identical and you confirmed that with the other player, piece by piece
-You deal the same damage on tyranid-dummies, just to confirm your loadouts are completely identical
-You both fire full-auto/single-auto and you both aim for the lower torso to avoid headshots
-You both have a latency of 60 or less

Click to expand...

60 or less? That is like. . . dreamland. Mystical land of acceptable pings.

Also, the dummies show different damage on their torsos depending on the color of the part you are hitting (maybe? I don't really know. Seems random to me).